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during each of Earth's ve mass extinctions, the
rst about 440 million years ago. Greenhouse
gases spiked naturally over the millennia, and
Aussie biologist Veron says massive spewing of
carbon dioxide during periods of heavy volcanic
activity was likely a big player in coral decima-
tion, notably the most recent mass extinction
some 65 million years ago. At that time, oceans
absorbed more and more of those greenhouse
gases from the atmosphere, causing ocean acid-
ity to rise. e lower pH---a sign of high levels
of acidity---ultimately thwarted the ability of
marine creatures to build their limestone shells
and skeletons.
In some oceans this acidification is once
again happening. e most vulnerable to acid's
the prospect for recovery is uncertain. e rela-
tively quick shi in the world's climate, scientists
say, appears to be devastating for reefs. In corals,
warming temperatures and increased exposure
to the sun's ultraviolet rays lead to a stress re-
sponse called bleaching---when the colorful al-
gae in coral cells become toxic and are expelled,
turning the host animals skeletal white. Fleshy
seaweeds may then choke out the remains.
Major bleaching in the Great Barrier Reef and
elsewhere in 1997-98 was linked to a severe El
Niño year and record-high sea-surface tempera-
tures---in some spots 3°F higher than normal.
Another round began in 2001 and again in 2005.
By 2030, some reef experts say, these destructive
episodes will occur every year.
Heat is also implicated in a 60-year decline in
ocean phytoplankton---the microscopic organ-
isms that not only gobble greenhouse gases but
also feed, directly or indirectly, almost every oth-
er living thing in the sea. Reef sh, too, respond
to warmer waters---sometimes with bolder, more
aggressive behavior toward both predators and
prey. Changes in sea level, either up or down,
have a dire impact as well, exposing shallow cor-
als to too much sun or drowning them in deeper
water, where they're hidden from the light.
A more immediate concern is massive ood-
ing in Australia that earlier this year sent huge
plumes of sediment and toxin-laden waters onto
the reef o Queensland. e full harm to marine
life won't be clear for years, but long stretches of
the Great Barrier Reef could experience disas-
trous die-o s.
And then there's the acid test.
Reef ecosystems worldwide took a pounding
THE REEF'S HISTORY IS A
CATALOG OF DISASTERS
FROM WHICH IT HAS
ALWAYS RECOVERED.
Drawn to the smell of a dead
sperm whale, a ten-foot tiger
shark arrives at the edge of
the reef to feast on oating
esh. Bits of food le unde-
voured will fall to feed the
reef 's tinier residents.
GALEOCERDO CUVIER